It is well-known that directional couplers are usually used to solve the problems relating to power splitting in many microwave circuits. With the development of mobile communication technology and satellite communication technology, for convenient carrying and moving, the miniaturization of the communication devices becomes more and more important.
Branch-line couplers are widely applied to microwave integrated circuits and monolithic integrated circuits. The conventional branch-line coupler, such as the 3 dB branch-line coupler is constituted of four quarter-wavelength lines. FIG. 1 shows the circuit configuration of a conventional 3 dB branch-line coupler of prior art. In FIG. 1, the length and width of the branch-line coupler respectively are 7.4 mm and 9.14 mm. However, the branch-line coupler occupies a large area of the printed circuit board (PCB). Therefore, a minimized and high performance 3 dB branch-line coupler would be preferable.